After potatoes are cooked, they develop a dark bluish-grey pigmentation known as after-cooking darkening. This pigmentation is due to a complex of ferric (Fe.sup.3+) ions with chlorogenic and/or caffeic acids. In freshly cooked potatoes, a colourless chlorogenic acid-ferrous iron (Fe.sup.2+) complex is formed, and is subsequently oxidized in air to the coloured chlorogenic acid-ferric iron complex.
Various factors are known to affect after-cooking darkening, and the role of organic acids, especially citric acid, has received the most attention from researchers. Juul (1949) showed that the discolouration was influenced by citric acid and he attributed it to a pH effect. Beta-Smith et al. (1958) recognized that citric acid has a chelating effect and stated that the distribution of blackening in individual tubers was governed mainly by the competition between chlorogenic acid and citric acid for iron. Hughes and Swain (1962) studied the effect of citric acid, orthophosphoric and malic acid on the colour of various phenol-iron complexes and concluded that citric acid was the most important of these factors in reducing the intensity of colour of the chlorogenic acid-iron complex. In 1963, Heisler et al. reported significant correlation coefficients between the degree of discolouration and the content of the citric acid in potatoes. Thomas et al. (1979) attributed the induction of after-cooking darkening in irradiated potatoes to decreased citric acid and increased polyphenols in the tuber tissues.
Many products have been reported to reduce or prevent after-cooking darkening in various foods. These include sodium acid pyrophosphate (SAPP) (Smith 1958; Smith and Davis, 1960, 1961, 1962a,b,c,d; Davis and Smith, 1964), ethylenediamine tetraacetic acid (EDTA) and its salts (Smith and Muneta, 1954), gluconic acid, citric acid, sodium gluconate, sodium citrate, ammonium gluconate, and sodium bisulfite (Smith, 1987). It is believed that these chemicals reduce pigmentation by sequestering or chelating the iron in the potato so that it is held in a nonionizable form and cannot take part in a reaction with chlorogenic acid, thereby preventing the formation of the dark coloured pigment. Fellers and Morin (1962) obtained a patent on the prevention of after-cooking darkening with several chemicals of the EDTA group.
Most french fry processors do not use any of the above chemicals, with the exception of SAPP which is now used routinely for the treatment of blanched potato products before frying and freezing. SAPP treated potatoes can develop a bitter chemical taste (Ng and Weaver, 1979; Smith 1987), and more recently it has been reported to be a concern to chemical additive-conscious consumers. In addition, SAPP is disadvantageous in view of its cost.
There exists a need to provide an improved process of achieving prevention of after-cooking darkening of french fries.
In spite of the apparent importance of iron and organic acids, the literature reports no attempt to simultaneously complex the chlorogenic with calcium ions (Ca.sup.2+) and the ferrous iron with an organic acid. Alternative products or processes to SAPP and other chelating or complexing agents are required.
Edible food coatings made from starches, gums, proteins, dextrin, lipids and derivatives have been suggested for the protection of other kinds of food products against moisture and oxygen (Guilbert, 1986; Gennadios and Weller, 1990). The use of dextrins, starches and gums in flavour encapsulation has been recently reviewed (Reineccius, 1990). It has also been reported that pullulan, a water soluble polysaccharide has oxygen-barrier and film forming characteristics, (LeDuy et al., 1988; Sugimoto, 1990). However, none of these procedures is entirely satisfactory.
A search has been conducted in the facilities of the United States Patent and Trademark Office with respect to the patentability of the invention and the following United States patents were cited as the most relevant art:
______________________________________ 2,031,243 3,051,578 2,212,461 3,063,849 2,448,152 4,767,630 2,322,516 4,818,549 2,893,878 4,911,940 2,894,843 4,988,523 2,987,401 ______________________________________
Several of the above patents disclose the use of calcium for pretreating potatoes. U.S. Pat. No. 2,031,243 discloses the use of a mixture of ionic species, including calcium, in cooking and various food applications; U.S. Pat. No. 2,448,152 discloses pretreating potato pieces with a calcium salt, typically calcium chloride, to avoid discolouration in potato chips; U.S. Pat. No. 2,893,878 discloses treatment of potatoes with calcium chloride to inhibit browning of raw potatoes; U.S. Pat. 2,894,843 discloses the treatment of freshly peeled potatoes with calcium sulfite to inhibit discolouration of the raw vegetable; U.S. Pat. No. 2,987,401 discloses treatment of peeled potatoes with calcium phytate to inhibit discolouration of the raw potato; U.S. Pat. No. 3,051,578 discloses the treatment of potatoes with EDTA and salts thereof, including calcium, for preventing greening in unpeeled potatoes; U.S. Pat. No. 3,063,849 discloses the production of dehydrated cooked potatoes by precooking potato slabs in water containing calcium ions; U.S. Pat. Nos. 4,818,549 and 4,911,940 disclose treating apple cubes with a solution containing calcium chloride; and U.S. Pat. No. 2,322,516 discloses the treatment of soya with various salts, including calcium acetate.
In addition, U.S. Pat. No. 2,212,461 discloses the preparation of french fried potatoes in which the potato strips are soaked for a prolonged period prior to cooking in a solution of acetic acid to avoid a non-uniform colour and burnt taste arising from high sugar levels. U.S. Pat. No. 4,767,630 discloses the production of sliced dried fruit or vegetable chips in which the chips are treated with a composition which includes an anti browning agent, which may be a variety of organic acids, prior to dehydration. U.S. Pat. No. 4,988,523 discloses a treatment of peeled fruit and vegetables with a medium of phosphate salts and citric acid.
The procedure of dipping blanched potato strips in calcium acetate solution in order to control after-cooking darkening of french fries in accordance with the present invention is not disclosed in any of the foregoing patents.